The weird thing is that the iwad contains reasonable-looking textures (SKY1 and SKY2, maybe even SKY2A) but it also has Doom II skys in RSKY1 and RSKY2...

Maybe those RSKY* textures don't belong in the iwad and are a result of a build script bug? Checking an original Ultimate Doom iwad shows only these textures: SKY1, SKY2, SKY3, SKY4, F_SKY1. An original Doom 2 iwad has these textures: RSKY1, RSKY2, RSKY3, F_SKY1.

Also, dumb question: what is F_SKY1 for? I never saw that texture used anywhere.